The present invention relates to a low temperature fired dielectric ceramic composition, and a method of making same. More specifically, it relates to a dielectric ceramic composition formed by firing a mixture of a base ceramic preparation comprising a mixture of dielectric oxides and a glass frit at temperatures not exceeding 1150.degree. C.
Multilayer ceramic capacitors are commonly made by casting or otherwise forming insulating layers of dielectric ceramic powder, placing thereupon conducting metal electrode layers, usually in the form of a metallic paste, stacking the resulting elements to form the multilayer capacitor, and firing to densify the material and form a solid solution of the constituent dielectric oxides. Barium titanate is one of the dielectric oxides frequently used in the formation of the insulating ceramic layer. Because of the high Curie temperature of barium titanate, however, strontium and zirconium oxides are commonly reacted with the barium titanate to form a solid solution, thereby reducing the Curie temperature of the resulting ceramic material. Certain other oxides, such as manganese dioxide, may also be added to control the dielectric constant of the resulting material by acting as a grain growth control additive.
Because the materials commonly used to produce ceramic capacitors with dielectric constants greater than 4500 are generally fired to maturity in air at temperatures greater than 1150.degree. C., the metallic electrode layer must be formed from the less reactive, higher melting alloys of the so-called precious metals, such as palladium and silver, palladium and gold, and other similarly expensive alloys well-known in the art. This is necessary in order to prevent either reaction of the electrode with the insulating ceramic layer or melting which might result in discontinuities in the conducting layer. A method of producing a ceramic composition with a high dielectric constant and other suitable properties, which can be fired at temperatures below 1150.degree. C., would permit the use of a less costly electrode material without sacrificing capacitor performance.
In the prior art methods for producing a low temperature fired dielectric ceramic composition, as set forth, e.g., in U.S. Pat. Nos. 3,682,766, 3,885,941 and 4,066,426, a base ceramic preparation of dielectric oxides is completely presintered to form a solid solution at a high temperature, ground to a fine powder, and mixed with glass frit. This ceramic-glass mixture can be fired at a substantially lower temperature than the base ceramic preparation alone, thus permitting the use of lower melting materials, e.g., pure silver or silver-palladium alloys having a substantially higher percentage of silver than the higher melting alloys previously used for the conductive metal layers. A major disadvantage of the prior art methods for making a low temperature fired ceramic capacitors, however, is the necessity for presintering the base ceramic preparation at a high temperature to form a solid solution of the constituent oxides before mixing with the glass frit. Another disadvantage of the prior art is the large volume of glass frit necessary to achieve the low firing materials.
Therefore, it is an object of the present invention to produce a ceramic composition with high dielectric constant capable of being fired at temperatures below 1150.degree. C. Another object of this invention is to produce a low temperature fired, high dielectric constant ceramic composition whose dielectric constant varies predictably with temperature. Still another object of this invention is the provision of a process for preparing a dielectric ceramic composition with high dielectric constant which eliminates the necessity for a presintering step at a temperature of greater than 1150.degree. C.